Heathrow airport is investigating after an unemployed man found a memory stick containing top-secret security intelligence including how the Queen is protected.

The unencrypted USB stick contained the timetables of anti-terror security patrols at the airport as well as maps of escape shafts linked to the Heathrow Express.

The stranger who found the USB stick dumped on the West London street plugged it into a library where he was shocked to discover the 174 files – many marked confidential or restricted.

According to the Sunday Mirror, Heathrow airport launched a “very, very urgent” internal investigation and said all of its security plans had been reviewed. Heathrow added it was “confident” the airport was secure”.

Some of the files disclosed the exact route taken by the Queen to the airport and the security methods used to protect her.

Other files told the types of ID needed to get into restricted areas and a map showing the CCTV cameras at the airport.

Another document focused on the type of terror threat facing the airport, the Mirror reported.

The newspaper said they alerted Heathrow and passed on the information.

In a statement, a spokeswoman said: “Heathrow’s top priority is the safety and security of our passengers and colleagues.

“The UK and Heathrow have some of the most robust aviation security measures in the world and we remain vigilant to evolving threats by updating our procedures on a daily basis.

“We have reviewed all of our security plans and are confident that Heathrow remains secure.

“We have also launched an internal investigation to understand how this happened and are taking steps to prevent a similar occurrence in future.”